Thrombin, the most potent activator of platelets, works by activation of the G protein-coupled protease activated[unreadable] receptors PAR1 and PAR4. These receptors initiate G protein signaling cascades leading to increases in[unreadable] intracellular calcium and secretion of autocrine activators, promoting platelet aggregation. Direct thrombin inhibitors[unreadable] such as bivalirudin are rapidly increasing in use in interventional cardiology. However, thrombin's direct role in[unreadable] coagulation, as well as its underappreciated protective role on the vasculature through the thrombin/thrombomodulin[unreadable] activation of Protein C, are blocked by this strategy. An alternate therapeutic target is the PAR receptor system,[unreadable] because selective blockade of thrombin's cellular signaling effects might further decrease bleeding risk. We have[unreadable] preliminary evidence that points to differential signaling through PAR4 compared to PAR1; the molecular signaling[unreadable] process through PAR4 receptors to inside-out integrin signaling, secretion and aggregation will be thoroughly[unreadable] investigated in Aim 1. In Aim 2, we will determine the molecular basis of PAR4 signaling with novel G protein[unreadable] blocking tools designed for studies in human platelets that will inhibit, one at a time, activation of Gq, Gi, Go, G12 or[unreadable] G13 signaling through PAR receptors.[unreadable] Although bivalirudin is increasingly used in acute coronary care, the effect of thrombin inhibition on platelet[unreadable] function has not been thoroughly investigated. Thus we propose two clinical aims to compare platelet PAR[unreadable] signaling and platelet function. In Aim 3, we will investigate patients with stable or unstable coronary artery disease.[unreadable] Their platelet PAR signaling to platelet function will be studied before and during bivalirudin in the Vanderbilt Cardiac[unreadable] Catheterization Laboratory. The second clinical.study aims to evaluate PAR signaling in metabolic syndrome and[unreadable] diabetes, conditions complicated by a high risk of atherothrombotic events. In Aim 4, we have designed a clinical[unreadable] study to address the contribution of PAR receptors to the abnormal platelet activation in these thrombotic states.[unreadable] These detailed mechanistic studies on human platelet PAR1 and PAR4 differential signaling, and studies on effects[unreadable] of bivalirudin on platelet function will provide insight into platelet activation in atherothrombosis, and has the[unreadable] potential to impact future antiplatelet therapeutic strategies.